How to maximize shaft retorting yields 30th Oil Shale Symposium - - PowerPoint PPT Presentation
Misting 101: How to maximize shaft retorting yields 30th Oil Shale Symposium October 17-20, 2010 Larry M. Southwick, P.E. Cincinnati, Ohio Outline Introduction Gas Combustion Process Misting Benefits and
How to maximize shaft retorting yields
October 17-20, 2010 Larry M. Southwick, P.E.
Cincinnati, Ohio
Introduction Gas Combustion Process Misting Benefits and detractions Conversion to “hard driving” Other units The solution
US Congress in 1944 enacted Synthetic Fuels Act authorizing construction of demo plants
Oil Shale Experiment Station at Rifle, Colorado
Retorting processes studied depended on method of heat application:
Thru wall - Pumperston Combustion in retort - Gas Combustion Heated gases or liquids - Royster Hot solids - TOSCO
USBM Bulletin 635 reported on Gas Combustion
Other studies by oil companies (6 Company, 17 Co)
Crushed and sized shale Rising hot gases retort shale
and vaporize oil
Carbonaceous residue is
burned in combustion zone
Oil product removed from
gas, which is recycled back to retort
Process works efficiently
because of mist formed in product cooling zone
“From the onset of experimental work, it was observed that the gas streams from the retorts usually contained shale-oil mist” (not droplets, but fine mist)
“This fundamentally new concept led to the development of Gas Combustion Process”
“If the oil is to leave the retort as a mist in the offgas stream, the droplets must be formed in the spaces between the shale particles and must be small enough so that inertial separation does not occur”
“A refluxing problem occurs when the amount of oil on the shale is great enough to drip or flow down through the bed of shale”
Entrainment of droplets off of shale does not occur here because gas velocity is too low - thus oil collected on shale will descend with the bed = REFLUXING
Mist forms just above
retorting zone
Retort operates as a
countercurrent heat exchanger
No sharp demarcation
between retorting and product cooling
Assume 700°F shale
temperature as dividing point
Refluxing of condensed mist causes oil cracking
Alters heat distribution in misting section due to revaporization and secondary cracking
Equilibrium is stable under refluxing and not-refluxing
Often depends upon conditions at start of run
Cracking produces lighter, less viscous oil, but loss of production is severe
Mist is formed if:
Oil vapor cools until gas becomes
saturated
Nucleation occurs
Supersaturation, S, favors mist formation
S is oil partial pressure in gas divided by its vapor pressure at shale temperature
Mist forms when heat transfer to shale exceeds mass transfer of oil to shale
Mass transfer depends on diffusion and
Nucleation sites help form mists
Mist flooding rate obtained by drawing mist from retort and feeding to external bed
Raise cooling rate by lowering
When MassMeanDia = 2.5µ
Oil rate is 8-10 lb oil/MSCF
Flooding mist MMD = 3.0µ
5-6 lb oil/MSCF Thus lb/MSCF 3-4 lost Thus there is a maximum carrying
capacity to gas
Flooding vel ¼ x 1” = 2.7 ft/sec
For 1” x 3” = 3.3 ft/sec
Refluxing caused by collision between mist and bed particle was incomplete explanation of refluxing – also unstable mist, mist growth and coagulation
Mist impactor is standard test
Stages, 16, 8, 4, 2, 1, 0.5 µ
Considerable (50%) collected in piping and elbows off retort
High dilution gas = oil loss from gas carrying capacity
Collection efficiency increases
Mist particle size goes up
Gas velocity increases
Mist loading increases
Small shale particles
High bed packing fraction (wide particle size range)
Distance above air inlet, ft. 8 6 4 2 Droplet, mass mean dia., µ 2.36 2.28 1.82 Plugged with fines Loading, lb oil/MSCF 9.36 8.04 5.61 Temperature, ºF 140 300 470 800
Once nuclei occurs, no new nuclei form
Mass balance confirms growth since larger diameter = more oil per particle = loading rate
So oil is growing on existing nuclei
Tests using injected nuclei did not resolve
Refluxing liquid would cause accretions to form just above air distributor, blocking retort operation
Use drawoff systems to collect refluxing liquid
Worked well on small lab retorts, 1”, 2”, and 3.6” Variable results when applied to 150 TPD retort Drew off at zone where shale temperature is 600 ºF
Two other options to eliminate refluxing
Draw off unmisted hot, dry gas
Draw off hot misted gas but above refluxing zone These point the way to “hard driving” of retort
Minimize losses from impaction of mist on shale particles - ergo no small particles
Maximize evolution of oil - ergo, the smaller the particles the faster the net retorting rate
Testing found that particles as small as 1/8 inch could be used, but particles smaller than that caused significant
Limiting the minimum size to greater than 1/8 inch provided no great advantage
But retort still limited by oil refluxing, not easily controlled nor readily amenable to design
The challenge was also what to do with the fines from crushing - ergo TOSCO process
Capacity of iron ore blast furnaces were increased 150 years ago by “hard driving”
Hard driving meant just feeding more and more ore
They found blast furnace could handle ~30X feed
So if remove shale retort bottleneck of oil refluxing, should be able to “hard drive”
Thus eliminate mist formation or oil condensation
6 Company (1966) solution was an oil drawoff pan, which did not work (a typical “boiling oil” solution)
Rather try one of the other two solutions not picked (pull
The oil would be cooled and condensed externally to retort in equipment similar to that used before
This now oil-free gas can be reheated, re-injected above pull-off, and provide mist-free shale heating
One shaft retort converted to this concept to make it operable
Original Hytort scheme ran under conditions where normal mists did not form (high pressure, H2 gas, small particles), and which enhanced condensation of oil vapor on solids
Thus extract fumes before they condense, inoperable otherwise
Scheme studied in cold flow model, had good zone isolation
Hot tests were always just with retort zone, which worked well
Spent shale gasifier had moving bed, rising vapors Hot shale still evolving gases - cools and condenses Mist or otherwise, refluxing became a problem
Zinc ore (oxide) reduced and volatilized from retort Zinc metal fume will condense upon contacting cold
downward flowing solids
These vertical shaft retorts extracted hot fume or mist Process on left used splash condenser, right had labyrinth
Shaft furnace, briquette feed (carbon + zinc oxide) Keep top of shaft hot (1000°C), so no mist forms Splash condensers inefficient, use four in series
Steel made in electric arc furnaces (EAF) by melting scrap Volatilizes zinc from galvanized steel, dust is hazardous EAF dust processed by heating to remove zinc Shaft furnace has similar zinc condensing problems
Nature of oil shale retorting leads to shaft retorts
Counterflow of oil vapor and cold solids leads to formation of very fine oil mists
Mists can lead to refluxing of oil, net yields suffer
Retort operation also suffers - accretions, flow blockage, channeling of gas and shale
Fines WILL lead to oil losses, lighter oil and more gas and more coke
Low top temperature can also cause yield losses
The bottleneck to capacity is oil refluxing down the retort
The higher the shale rate, the more likely refluxing will occur, which sets and limits the feed rate
Eliminate the oil refluxing bottleneck by removing mist or hot vapor before oil condenses onto shale
Collect oil in devices similar to WetTop operation, then reheat and re-inject gases above drawoff
Blast furnaces, zinc retorting and distillation, EAF dust processing, even modified Hytort concept provide examples
Further, if shale feed is wet, the heat required to vaporize the water can be supplied by the re-injected gas, eliminating high temperatures in the retorting zone
Shale feeding and withdrawal devices may have to be modified for the greater throughput